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In Reply to: RE: Progress posted by emailtim on July 12, 2017 at 10:46:55
They're a 30-year-old pair of Tympani IVA's that I bought used a few years ago for embarrassingly little, considering how amazingly good they are. I've been doing some repairs in a desultory fashion and I'm planning to do Satie's mod to replace the old midrange panels with BG Neo 8's, which is why I have the grille cloths off.
Here they are lined up:
But they block the projection screen if I do that, so I'm using them in split configuration, with the woofers behind the midrange/tweeter panels.
I did paint the EPS diffusers. They really soak up the paint and it's amazing how much heavier they get. :-) I think they sound better than they did before they were painted. Not only do they become more reflective but the paint dampens a resonance, if you tap them you can hear that they're acoustically deader and less likely to color the sound. I think wooden diffusers would sound even better, but the EPS ones are really easy to move around and as I said I'm probably going to make some transparent ones anyway.
There's no need for a central diffuser since they're actually located right at the first reflection points of the midrange-tweeter panels. Here's a photo taken from my listening position with a mirror, you can see that the back of the mid-tweeter panel centered in it:
Anyway, based on my brief listeningg session anyway, I can't recommend diffusion enough. The difference in imaging was night and day.
If I had a huge listening room and could pull the speakers way out from the wall, it wouldn't matter, but I can only pull mine out 5' or so and without the diffusers, the image was just too flat. It actually sounded more lifelike when I was listening off axis. With the diffusers, I'm getting much more depth and it feels like I'm in a concert hall, the magic carpet effect. And that's with the speakers closer to the wall than I'd been running them -- I'd really like to be able to push them back a bit because the room is so small.
I think based on my brief listening that the diffusion also improves the ambiance of the room, since the backwave is being scattered and randomized. I have less of an image-just-between-the-speakers effect and a more spacious, natural presentation.
I also listened to a piano recording last night and that too was pretty spectacular, as in it felt like I was in the hall with the piano on stage in front of me.
I need to add some absorption to the room too, it's still pretty bare right now.
Follow Ups:
Agreed. I use a pair of Skylines although I should likely get another pair for optimum results.
I wonder how much of a difference a 2 x 4 arrangement actually makes. It's what you usually see but how much does that have to do with acoustics, given that well depth and diffuser width also matter? And how much is just accommodating listeners at varying heights?
Have you tried comparing the skylines to QRD's? I'd think the skylines would be more effective.
Not tried the QRD flavor.
Mike Lavigne uses a "triple stack". :)
Mike's room is definitely making me feel ambitious. :-)
I wonder why he has them mounted in the center, rather than at the first reflection points? Also why he uses skylines in front and what look like horizontal + vertical QRD's at the sides?
I'm curious about pyramidal diffusers right now, because of that window -- I could make them easily with plexiglass. From what I've seen they're more deflectors than diffusers, in that they create specular reflections off to the sides. But that would allow me to create an RFZ, which I think would be ideal. I may model them with styrofoam to see how the effect compares to that of the QRD's.
I'm thinking that if you bent the sheets a bit, you'd get both reflection and a bit of diffusion . . . and if you sealed them and put some foam at the top and bottom you'd also get some bass trapping.
That is what I have read years ago in my acoustics research reading: Absorption for the first reflection points and diffusion at the centre. What an amazing room, and when Rives goes in there and builds the room you know it is done right. Notice also the curved front wall on which the QRD is ousted which scatters the waves as well.
Was there a link to the room?
Anyway, I think the requirements for dipoles are somewhat different since they radiate little to the first reflection points at the sides, and a lot to the first reflection point at the rear and also the second reflection point around the corners. So basically the treatment ends up behind.
I'm uncertain about absorption. The best way to build a room is with a(n early) reflection-free zone, no question about that. In a studio, you do that by angling the walls so the early reflections are directed away from the mixing desk. Then you put diffusion on the back wall of the studio to provide a pleasing sense of ambiance.
But we don't usually have the option of angled walls, and rear wall has to be a minimum of 8' and preferable 12.5 feet (20 msec round trip) behind the listener to use diffusion there. So usually what the acoustics guys suggest for a small listening room is absorption at the first reflection points, and also behind the listener.
The thing is, I'm concerned that a room like that will end up sounding dead and studio-like, which is to say clinical. Two channel needs some room ambiance. Right now, my room is too live and zingy so I do need some broadband absorption in there but I'm afraid that absorption at all the first reflection points will be too much.
Another issue is that I can't put absorption over the window without blocking the window. There is some transparent micropore absorption stuff but I don't think it's as effective as fiberglass.
At some point, I'll probably get some 703 and experiment with absorption on the front and rear walls to see how it compares to the diffusion . . .
I think your sliding panels are an excellent idea. Have a sliding absorber. When things need to get critical slide it over the window, if you are like me your eyes are closed anyway. Or maybe one that flips up to the ceiling like a Murphy bed. Hey, I am giving myself ideas!
I thought about making shutters that would flip over the window from the sides . . .
Here's some commentary.
"...the room is purpose designed as a 2 channel listening room by Chris Huston of Rives Audio. the below picture does show 2 elements added since the room was built in 2004. the RPG Skyline diffusers (3 stacked) between the speakers, and the Auralex T-Fusors on the side walls next to the speakers.
I guess we'd have to ask Rives. Presumably, they know a lot more about it than I do! But the last thing I want to do is bring an acoustician in and actually make my room sound good. :-)
Edits: 07/15/17
the Rives Audio website has been gone for a while...
I'm learning a lot though - that diffuser's like the EPS are also pretty good attenuators at frequencies where the reflected wave forms cancel because of phase delays - a 3 inch deep reflector difference cancels a -1000 Hz signal very effectively. I've used these kinds (homemade) a lot, on sidewalls and they work pretty good as much as an attenuator as a diffusor. (as measured using a room acoustics software program (DIRAC). I think that placement directly behind a planar would not be as good as the placement you show. There are some ASTM papers discussing the performance of these kinds of diffusors that are generating up to 15dB attenuation at perpendicular incidences at some frequencies.
"Fancier" (more random) diffusors (Acoustics First) shown in the picture still attenuate but not in so much a "peaky" way.
"The hardest thing of all is to find a black cat in a dark room, especially if there is no cat" - Confucius
I'm OK with some degree of attenuation, in fact I think it's probably desirable since early first reflections have to be about 20 dB down from the direct sound before they stop influencing the image and diffusers are kind of marginal for that. Not wild about the notches -- I've read before that QRD's have those issues -- but I'm not sure how sonically significant they are since it's all happening above the Schroeder frequency and at lower levels as well.
I didn't know that those Acoustics First diffusors had better performance than a QRD (or presumably skyline) but it makes sense since they aren't discrete. They wouldn't work for my application, though, because of the window. I'm curious now about old-fashioned pyramidal diffusers, which are really more reflectors than diffusers since they produce lobes off to the sides -- basically a few specular reflections. Ideally, I'd like to have a compact way to make an RFZ zone. You need as I recall about a 30 degree angle to do that and I'd contemplated making a Fresnel mirror but that generally requires acoustical stuffing -- if I used a lucite prism shape it could be entirely transparent, perhaps some damping material at the top and the bottom within the prism.
Great score and thanks for the feedback. You have a lot of fun ahead of you. They look like they need a lot of juice for 3 panels per side and appear pretty wide. You say your room is small, but it appears at least wide. Can't judge the depth. Is your front sound stage using the longest or shortest wall ?There are some DIY diffusers floating around. I am debating on building an EPS version based on a QRDude design or a wood version based on the LeanFuser project. I have read both wood and EPS versions benefit from being sealed to make them more reflective.
I didn't think about the paint dampening the EPS versions, but it makes sense. There are some paint hardener additives that should also help. I have also read that some locate diffusers over bass traps. If I make some, I will try placing them in the corners over the bass traps.
Edits: 07/12/17
PS forgot to include the room size. Front-back it's 13' or 17' -- there's a big entrance arch that opens up onto the hall. So L shaped. That 17' part really helps with the bass since otherwise the room would be almost square, but it's over a bit too far to benefit my listening position.
I actually considered knocking down the wall -- this room was originally much bigger but they built a small bedroom in one corner. Obviously a major project, but it would become a much better room if I did.
The wall the speakers are against is 14'-6" so it's the long wall if you use the 13' dimension for depth.
Originally I'd hoped to put the speakers flanking the fireplace but the mantle screwed up the sound and I would have had to do major surgery to flush the wall out and fix that so I went back to the sideways arrangement.
At some point, I'm going to try putting some broadband absorption behind my listening chair, the usual recommendation if you're sitting against a wall . . .
Almost exactly my room's dimensions---13' deep, 14'4" wide, putting the panels "on" the long wall, firing across the shorter dimension. I could reverse that, but I like the extra width to get the Tympani panels further apart if I don't stagger them, and because of the window and door locations, which are symmetrical this way.Speaking of doors, there are two on the wall the speakers face, one to a bathroom, the other to a walk-in closet (the room is the master bedroom in the house). I can experiment with having them closed, open, or partially open. I assume the bathroom door should be closed for listening---who wants to listen to the echo of tile?! It has occurred to me that the closet could be modified to act as a Helmholtz resonator, installing a limp mass in place of the door!
Remind me again---in Tympani-IV's and IVa's, which bass panel is deep bass and which is more midbass? The one with tuning dots is midbass? The one with the terminal plate having the connectors is which? I remember Satie (I think it was) saying if those panels are placed right up against the side walls, the deep bass panel should be closest, rather than the midbass panel. Am I remembering correctly? Thanks all---Eric.
Edits: 07/18/17
Yes, I place the deep bass panel at the wall when they are separated from the MT and placed against the wall for wall loading. That placement with the bass panels facing dead forwards towards the midline of the room provides very powerful deep bass given sufficient power. In that scheme you minimize the dipole cancellation on one side and capture the room's tangential mode which is the deepest bass mode of your room while the side to side mode is minimized so that your bass is a bit more uniform. Add rigid bracing and you can play your organ spectaculars convincingly.
Love your closet Helmholtz resonator idea. Actually you can do that with walls too, build them as inexpensive bass traps if you're building a room from scratch.
For doors, I tend to think it sounds good to break up the regularity of the room. That reduces modal problems (my woofers sound a *lot* better firing into the L-shaped part of my room) and also delays reflections which is almost always good in a small room. Bathrooms make good echo chambers, I'd try it with the door open and closed and just go with the one you like and not worry too much about how accurate it is because 2 channel stereo just isn't accurate, it can't be, it needs to take advantage of your room acoustics.
The center panel is the deep bass panel. This provides maximum baffle width where it's needed most. But it isn't really quite as simple as that, my IVA's have their second lowest resonance section on the midbass rather than the bass panels, presumably to take advantage of proximity to the floor.
Satie's right that if you separate the bass panels you should put the deep bass panel against the wall so that the wall becomes in effect a baffle extension.
Hey, has anybody tried filling the gap between the woofer panels and the ceiling to create a 1D infinite baffle?
Tympani IV has three buttons on the mid bass driver, none on the low bass. Connection plate is on the low bass section.
Tympani IVa has three buttons on the mid bass and one on the low bass driver. Connection plate on the low bass.
Great, thanks all.
I didn't think about paint hardener!
I think you'll get the most bang for your buck from putting the diffusers at the first reflection point, those first reflections rob the image of depth since the ear determines the size of the space by comparing the arrival time of the direct sound and the first reflection sound.
Corner diffusers would probably be second, in that case you have a second reflection off the front wall to the side wall and to your ears. That's also an early arrival. You can use a couple of mirrors to determine where you want to put the diffusers. Putting them over bass traps might work but only if the reflection is in the same place.
Diffusion also helps elsewhere in the room, it can break up slap echo and also it's often used on the rear wall to provide a nice sense of ambiance and also increase the apparent size of the space.
There is a thread on the Gearslutz site where Jens Eklund (who makes EPS diffusers) posted some graphs showing that they need to be paint to work best.
I found this out personally, having used some unpainted QRDs for a while before painting them. It makes a difference. Check out page 8 on the "first diy diffuser build" sticky on the Audio Circle Acoustics section (my moniker is jk@home there), on how I painted them with foam brushes. I use three across the front wall, and have posted them here before.
As far as corner diffusers, I drilled up some diy BAD panels and placed them on my back bass trap, out of the reflect free zone. Pic above. The idea is to have the speaker output hit the BADs, then forward to the QRDs on the front wall, then to the listening position.
LOL, John, this is going to be sort of a "relief" to my wife. On our first date, like 40 years ago, I took her to a classical music concert, her first in such a hall. I joked that the color on the walls was "acoustical". She bought it, but only for a while. Nowadays, I get punched in the arm for even bringing it up.
My listening room is in an upstairs spare bedroom, my wife rarely goes in there. But when she does, boy does she notice any "changes" :) First thing that came out of her mouth when she saw the BAD panels..."how long did you take to drill all those holes!".Congrats on 40 years, we hit #39 this July.
Edits: 07/27/17 07/27/17
Congrats to you, too! We actually hit 39 also, back in May. We dated for a year or so, and that's how it all began. (An invitation I never intended to make and about the funniest evening of our lives.) You know, that's encouraging. I just learned from a client of mine that they also hit 39 last week.
Anyway, since we share the same system -- dual HT/music -- modding the Maggies and system raises issues in this apartment. Largely, they won't cook me dinner while the mess lasts. I am only getting a break now because HER new TV messed up MY stereo. So I have a couple of weeks or so more to fix the sound, even as work keeps requiring more hours. Slowly but surely, I am starting to enjoy the system's sound again but at least she's already happy with the "John Wick 2" gunshots from the Maggies. That gets me a decent dinner, for now. :)
I feel your pain, our HT system is in the living room, which is my wife's domain. Last fall I built and installed a couple new subwoofers, using the Parts Express Ultimax 18" drivers and flat pack boxes. The combined weight of box and driver is over 100 lbs., so the drivers had to be installed close to where the subs would reside.Luckily she was out of town for two consecutive weekends, which gave me just enough time to get the job done, without the added inconvenience to her.
Edits: 07/28/17 07/28/17 07/28/17
Damn, I keep meaning to answer but follow the links and get sidetracked. I was intrigued by the dual use of diffusers for bass trapping and followed to another thread but it got bogged down in a long (though not uninteresting) discussion of floors . . .
Anyway, really impressive BAD panel. How long did it take you to make them? That's a lot of drilling. I see you bent it too for better diffusion.
An interesting approach overal. I thought about putting some more number theory diffusers there but not sure how that would affect corner bass trapping.
Since one of the issues I have is that my diffuser goes over the window and I want to be able to work in my listening room which is much nicer with the window unblocked, I've been thinking about using plexiglass triangular diffusors at the first reflection point. That would give two lobes off to the sides and I think ideally those would hit diffusers. But it's hard to know what the angles would be at this point since I haven't finalized speaker location.
It took 6-7 hours, not counting breaks, to do the final panel drilling. But that was after quite a few false starts, and many hours wasted. Started out with 1/4" HDF from the Home Depot, but ended up reusing some Poly diffusers that were being stored in the attic (pic above). Thanks to the heat in the attic, the bend was built in :)
I had originally bought these for the front wall corner bass traps, due to my MMGs firing into the traps and losing the backwave. If you didn't want to go the diy route, some RPG BAD panels may work there also.
Here's a link that goes into more detail:
That's really great. Read through the threads -- I like the bit about blowing up the PDF file!
I'm still not sure what I'm going to do. All of the acoustical advice says absorption is the best way to get an RFZ in small rooms, but I'm afraid that if I put absorption at all the first reflection points it will sound too dead -- plus I don't want to cover up any windows. So I'm still thinking of making a transparent diffuser for the first reflection points behind the speakers -- which would have to be either a polycylindrical, a triangle (seems easiest), or a 1D number theory diffuser -- I don't think a transparent skyline diffuser would be very effective! But BAD panels would be a possibility for the corners.
Yeah, windows are a challenge. Not familiar with the triangle diffusers. There are some ideas for windows on this GIK thread below. I like the idea of adjustable wood blinds. Not acoustically perfect, but should be better than plain glass.
Spent some time this morning reading through the thread and following up some of the leads.
I like the blind idea, but I fear they wouldn't be very good at diffusion -- they'd be deflectors rather than diffusers because they lack the random pattern element of a number theoretic diffuser.
That's true in a way of a triangle diffuser or it's 2D cousin the pyramidal diffuser. A triangle diffuser basically creates a null in the specular direction and has a lobe off on either side. So it's useful for creating an RFZ and (best of all) would be easy to build out of a couple of sheets of plexiglass, as well as not damaging the view. But the lobes themselves aren't diffuse -- basically two specular reflections, each from an angled plane.
At some point I think I'll ask my question there, in that thread, or another. I would be nice if someone made something commercial as they can do things that I can only do with difficulty, like bending plexiglass or molding transparent QRD's or PRD's!
Those transparent absorbers look interesting too . . .
If you could find a clear thin flexible plastic, that could be bent into a curve for a polycylindrical, that would be doable. You could first try 1/8" HDF to get an idea of the sonic results.
I was thinking about that, did some research on the bending radius of polycarbonate, then when I was at Home Depot the other day did some experiments bending the polycarbonate and plexiglass sheets.
One thing I noticed is that plexiglass is much less active acoustically so it seemed a better candidate. But I'll have to check the bending radius. I know from experience that thin sheets of plexiglass are themselves a lot more active acoustically than thick ones.
It occurred to me that I could laminate several thinner sheets to make a thicker one. Also that I might be able to make some kind of constrained layer damping arrangement. Vinyl is transparent and pretty well damped, so what would happen if I took two thin sheets of plexi or polycarbonate and sandwiched a layer of vinyl between them? I smell a patent. :-)
I was also wondering if I could do bass trapping by putting damping material inside the diffuser above and below the window opening so it wouldn't block the view. You'd end up with a poly that had a transparent section where the window was but damping material above and below it. I smell another patent. :-)
Also, if you used that transparent micropore sound absorbing material you could put it behind a perforated sheet and make a transparent BAD panel or scattering sheet.
Sounds like a fun project. From my research, the go to forum for poly info is Gearslutz, so do a search there.
Alas, I've been reading up on polys in Cox and D'Antonio and it looks like they just don't have sufficient diffusion to meet the -20 dB criterion for first reflections, particularly when the semicylinder is flattened. Even the optimized curves don't quite make the grade.
So I'm back to square one -- slanted planes, triangles, or a transparent Schroeder diffuser . . .
How do they compare to the more familiar QRD diffusers?
The number theory diffusers I've looked at seem to be more effective but some of that may be due to absorption. In general,
- Number theory diffusers are more effective at some frequencies than others and can exhibit lobing, which at a minimum means that they have to be listened from 8' or 10' away (depending on low frequency diffusion extension).
- Number theory diffusers tend to do poorly at high frequencies although more elaborate designs like fractal diffusers address this. Semicylindrical diffusers don't have an upper frequency limit.
- A flattened semicylinder has issues at oblique angles and polycylindricals start to exhibit lobing, which can be minimized by modulation schemes, just as with number theory diffusers.
- Number theory diffusers affect both amplitude and phase, whereas polycylindrical diffusers affect only amplitude. It's generally desirable to randomize phase in a diffuser.
- Some say that polyclyndrical diffusers sound harsh, perhaps because of comb filtering?
- In general, it's hard to get the -20 dB attenuation you want at a first reflection point with a <20 ms delay with diffusion alone. -15 dB seems to be more typical for a number theory diffuser and the polycylindricals seem to be worse than that.
So the practical limit is having about 15db attenuation of the reflections? Meaning that you need to get the rest of the attenuation via raw distance or added absorption. But getting 10-15 db attenuation is definitely a good start with such low cost diffusers as you are using, which diffuse both on attenuation and phase?.
Hard to say, as the behavior of Schroeder diffusers is somewhat complex, both as a function of frequency and angle. There are frequencies at which they don't work at all. But I'd say that 15-20 dB is pretty typical of the polar curves that I've seen, with lobing that pushes it above or below at various angles.
The inexpensive diffusers really aren't that different from the expensive ones of comparable size as far as performance (as opposed to appearance and robustness) is concerned, if you paint them, anyway, to minimize absorption (which who knows might not be desirable at an early reflection point where you're going after 20 dB). All of the Schroeder diffusers (QRD, PRD, optimized, finned or finless, etc.) affect both amplitude and phase because they work by introducing pseudo-random delay, leading to phase cancellation or reinforcement. A geometric diffuser like a polycylindrical or triangle doesn't alter phase.
The one exception to that I know of is that the fractal designs have a wider frequency range. And of course this all assumes that the mathematical design is good in the first place. But really, once you've chosen the desired optimized design, the material doesn't make much of a difference and they all have similar constraints.
"So I'm back to square one -- slanted planes, triangles, or a transparent Schroeder diffuser" . . .Like this?:
Edits: 08/04/17
Brilliant!
Looks like it's only effective to 4K or so, though.
I'd thought about making a leanfuser out of plexiglass, it would be similar to this and not that hard to make (I think polishing the edges would be the time consuming part), but it also has bandwidth limitations. There's a fractal version but it wouldn't be practical to make the smaller parts by hand. However, it occurred to me that it would be possible to cast them out of resin and then glue that on the acrylic.
Gary seems to have his bases covered (literally) =).
FWIW, I was considering this latex paint hardener additive.
Whoa, yeah! I'm not sure that that buys you anything but it sure looks cool. :-)
Latex paint additive looks cool too -- though is a bit of absorption really all that bad? Depends on your room, I guess, and how much HF damping there is.
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